Although applicable to cooperative networks in general, the embodiments of the present invention will be described with regard to a vehicular cooperative network.
Cooperative vehicular networks or systems are used to increase road safety and traffic efficiency. Such cooperative vehicular networks may be based on wireless communication based on IEEE 802.11 wireless LAN technology among vehicles and between vehicles and a roadside infrastructure, for example roadside infrastructure nodes according to IEEE 208.11p and/or its European variant according to ETSI EN 302 663 “Intelligent Transport Systems” (ITS).
In cooperative vehicular networks the stations are vehicles, i. e. highly dynamic nodes with challenging propagation conditions. For a communication between vehicles a dedicated spectrum in the 5 GHz range was allocated. In a scenario with many nodes and with a high data rate from corresponding applications using these communication channels the communication channels can easily be saturated. This saturation leads to an unreliable communication between the vehicles and therefore in an inefficient operation of the cooperative vehicular network. In saturated conditions, the time for accessing a communication channel is significantly increased and the probability of packet reception is decreased at all distances or in other words packet loss is increased.
Due to a lack of coordinating infrastructure in cooperative vehicular networks cooperation has to be performed in a decentralized manner. For example for safety reasons vehicles may send periodic status messages to advertise their presence to other vehicles. These periodic messages opposed to event-driven messages are the basis for many safety applications like electronic break light, etc. and they may contribute considerably to the load on the wireless communication channels.
In order to control the load on these wireless communication channels the transmit power respectively output power of stations/nodes, here vehicles, in the cooperative vehicular network may be adjusted according to the actual load on the communication channels in the cooperative vehicular network. By decreasing the transmit power of a sent packet, this also reduces the spatial coverage and hence the load at a particular location in the communication range of the vehicle sending out the packet. Further conventional options to control congestion on the wireless communication channel include for example adjusting the packet generation rate, the carrier sense threshold or a combination of both of them.
For adjustment of the transmit power a so-called transmit power control TPC was proposed for congestion control by ETSI TS 102 687 and further D-FPAV, which was defined in Torrent-Moreno, M.; Mittag, J.; Santi, P.; Hartenstein, H., “Vehicle-to-vehicle Communication: Fair Transmit Power Control for Safety-Critical Information”, IEEE Transactions on Vehicular Technology, vol. 58, no. 7, pp. 3684-3703, September 2009. (DFPAV), also available in M. Sepulcre, J. Mittag, P. Santi, H. Hartenstein, and J. Gozalvez, “Congestion and Awareness Control in Cooperative Vehicular Systems,” Proceedings of the IEEE, vol. 99, no. 7, pp. 1260-1279, July 2011.
D-FPAV is a transmit power control algorithm achieving congestion control under so-called fairness constraints. The term “Fairness” may for example be defined as in M. Torrent-Moreno, P. Santi, and H. Hartenstein, “Fair sharing of bandwidth in VANETs,” in Proceedings of the 2nd ACM international workshop on Vehicular ad hoc networks (VANET), 2005, pp. 49-58.
Congestion control in the wireless communication channel is according to D-FPAV achieved by exchanging of neighbour position information piggybacked in extended beacon signals from the vehicles. The added control information causes overhead scaling with the number of neighbour nodes/vehicles. Another alternative to reduce the overhead is to estimate the node density around every node and exchange a constant-size histogram of the node density in road segments as proposed in Mittag, J.; Schmidt-Eisenlohr, F.; Killat, M.; Harri, J.; Hartenstein, H., “Analysis and Design of Effective and Low-Overhead Transmission Power Control for VANETs”, Proceedings of the fifth ACM VANET 2008 (DVDE/SPAV).
Further conventional methods defining a packet rate control opposed to transmit power control for example available in the documents of H. Busche, C. Khorakhun, and H. Rohling, “Self-Organized Update Rate Control for Inter-Vehicle Networks,” in Proceedings of the 7th International Workshop on Intelligent Transportation (WIT 2010), 2010, of J. B. Kenney, G. Bansal, C. E. Rohrs, “LIMERIC: a linear message rate control algorithm for vehicular DSRC systems,” Eighth ACM international workshop on Vehicular inter-networking (VANET 2011), pp. 21-30, 2011, of C. Sommer, O. K. Tonguz, and F. Dressler, “Traffic information systems: efficient message dissemination via adaptive beaconing,” IEEE Communications Magazine, vol. 49, no. 5, pp. 173-179, May 2011, of M. Sepulcre and J. Gozalvez, “Adaptive wireless vehicular communication techniques under correlated radio channels,” in Proceedings of the 69th IEEE Vehicular Technology Conference (VTC Spring), 2009, pp. 1-5 and of T. Tielert, D. Jiang, Q. Chen, L. Delgrossi, H. Hartenstein, “Design methodology and evaluation of rate adaptation based congestion control for Vehicle Safety Communications,” Vehicular Networking Conference (VNC), 2011 IEEE, pp. 116-123, 2011.
Further conventional methods use an adaptation of the clear channel assessment (CCA) threshold, i.e. the threshold to detect and decode an incoming frame, for example described in the document of R. K. Schmidt, A. Brakemeier, T. Leinmüller, F. Kargl, and G. Schäfer, “Advanced carrier sensing to resolve local channel congestion,” in Proceedings of the Eighth ACM international workshop on Vehicular inter-networking (VANET 2011), 2011, pp. 11-20.
Even further other conventional methods combine transmit power control and rate control for congestion control of the wireless communication channel, for example as described in the documents of R. Baldessari, L. Le, W. Zhang, A. Festag: “Joining Forces for VANETs: A Combined Transmit Power and Rate Control Algorithm”, 7th International Workshop on Intelligent Transportation (WIT 2010), March 2010. (Combined rate and power control), of L. Le, R. Baldessari, P. Salvador, A. Festag, and Wenhui Zhang, “Performance Evaluation of Beacon Congestion Control Algorithms for VANETs,” in Global Telecommunications Conference (GLOBECOM), 2011, pp. 1-6, of C. Khorakhun, H. Busche, and H. Rohling, “Congestion Control for VANETs based on Power or Rate Adaptation,” in Proceedings of the 5th International Workshop on Intelligent Transportation (WIT 2008), 2008, of M. Sepulcre, J. Gozalvez, and H. Hartenstein, “Application-Based Congestion Control Policy for the Communication Channel in VANETs,” IEEE Communications Letters, vol. 14, no. 10, pp. 951-953, 2010, of M. Sepulcre, J. Mittag, P. Santi, H. Hartenstein, and J. Gozalvez, “Congestion and Awareness Control in Cooperative Vehicular Systems,” Proceedings of the IEEE, vol. 99, no. 7, pp. 1260-1279, July 2011 and of C.-L. Huang, Y. P. Fallah, R. Sengupta, and H. Krishnan, “Adaptive Intervehicle Communication Control for Cooperative Safety Systems,” IEEE Network, vol. 24, no. 1, pp. 6-13, 2010.
These conventional congestion control methods may be classified into proactive, reactive and hybrid methods. Reactive methods use information about the general congestion status based on local information or remote information transmitted from other nodes. Proactive methods estimate the transmission parameters that do not lead to congestion. Hybrid methods combine proactive and reactive methods.